The present invention relates to a conductive ceramic sintered body containing two types of conductive materials having different electric resistivities, the balance of which is essentially composed of silicon nitride and/or sialon.
Silicon nitride-base ceramics, such as silicon nitride based on .alpha.- or .beta.-Si.sub.3 N.sub.4, .beta.-sialon shown by the formula: Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0&lt;z.ltoreq.4.2), .alpha.-sialon shown by the formula: M.sub.x (Si,Al).sub.12 (O,N).sub.16, wherein x&lt;2, M represents Li, Mg, Ca, Y, or a rare earth element except for La and Ce, and their composite compounds, have excellent high-temperature strength and oxidation resistance, small thermal expansion coefficient and extremely good heat shock resistance. Accordingly, they have been being used for various applications in recent years. They are, however, extremely difficult to machine. One way to machine such ceramics is a discharge machining method, and various proposals were made to enable their discharge machining by imparting electric conductivity to them (See, for instance, Japanese Patent Laid-Open Nos. 57-188453, 57-200265, 59-207881, 60-33265, etc.) In addition, it was proposed to use such conductive ceramics as heater materials (Japanese Patent Laid-Open Nos. 60-60983 and 62-140386).
In the silicon nitride or sialon having electric conductivity, carbides, nitrides, etc. of transition metals in Groups IVa, Va and VIa of the Periodic Table are used.
However, most of these conductive materials have an electric resistivity on the level of 10.sup.-5 .OMEGA..cm. For instance, electric resistivity is 4.0.times.10.sup.-5 .OMEGA..cm for TiN, 1.6.times.10.sup.-5 .OMEGA. .cm for TiC and 1.8.times.10.sup.-5 .OMEGA..cm for ZrN. Therefore, an electric resistivity of as high as 10.sup.-3 .OMEGA..cm or more cannot be obtained without reducing the amount of conductive materials. However, since this makes it impossible to form sufficient paths for electric conduction in the sintered body, slight variations of the contents of the conductive material lead to drastic changes in the electric resistivity of the sintered body. As a result, it is extremely difficult to obtain a sintered body with stable electric resistivity which does not suffer from large variations. Incidentally, the variations in the electric resistivity are likely to appear among production lots, sintered bodies, and even in a single sintered body.
The ceramic sintered bodies disclosed by Japanese Patent Laid-Open Nos. 57-188453, 57-200265, 59-207881 and 60-33265 have an electric resistivity which tends to vary widely at high electric resistances, and no measure has been proposed to eliminate such variations.
On the other hand, conductive compounds such as carbides, nitrides, etc. of transition metals in Groups IVa, Va and VIa of the Periodic Table generally have high melting points, so they are less susceptible to deterioration even by high-temperature sintering. However, these compounds are inferior to the silicon nitride or sialon with respect to oxidation resistance. Accordingly, their maximum use temperatures are inevitably restricted.
As described above, the silicon nitride or the sialon having electric conductivity are excellent ceramic materials, but they are not free from the above-mentioned problems.